1. Field of the Invention
The present invention relates to a system for producing a hologram, and more particularly to an apparatus for producing a dot matrix hologram, which can speed up the production rate and reduce the bulk of the production apparatus. Furthermore, the resolution can be adjusted as the dot matrix hologram is being produced.
2. Description of Prior Art
Refer to FIG. 1, the production of a conventional hologram, such as a rainbow hologram, normally comprises the steps of: emitting a laser beam 2a from a laser light source 2; splitting the laser beam 2a into two laser beams 5a and 5b by a beam splitter 5 after passing through a shutter 3 and a mirror 4, wherein the beam 5a functions as a reference beam that is projected on a recording substrate after passing through a mirror 6, a spatial filter 7 and a lens set 8, and the beam 5b illuminates the object 17 after passing through a beam splitter 11, mirrors 12, 14, and spatial filters 13, 16, thus scattering onto the recording substrate 18; and exposing the recording substrate 18 by the interference fringes of the reference beam 5a and the object beam 5b. However, this conventional production method demands a high quality of laser beam, and is also costly and difficult to practice. Furthermore, the area required for constituting such a production apparatus is large since a large image area is required.
In the prior art, another type of hologram, a dot matrix hologram, has been provided. The fabrication of dot matrix hologram is similar to the printing process of a computer via a dot matrix printer, which prints an image outputted from the computer onto a document dot-by-dot. An apparatus for producing a dot matrix hologram functions as a dot matrix printer, converting an image, such as a fish's image, into a dot matrix of pixels via a computer, and exposing a recording substrate by means of the apparatus corresponding to the dot matrix of pixels, so as to transfer plural tiny areas of grating 20 to the substrate 22 and thus form a dot matrix hologram as shown in FIG. 2.
Since the apparatus for producing a dot matrix hologram is controlled by a computer, therefore it is easy to operate. Furthermore, since the exposure area is tiny, the exposure time can be reduced to a few microseconds; the optical system is thus less subject to vibration or interference from the surroundings. The stability of the apparatus for producing a dot matrix hologram is superior to the other conventional fabricating apparatus, thus reducing the cost.
A dot matrix hologram can present an image with a variety of bright and dazzling visual effects by changing the direction and density of the gratings formed in the plurality of tiny areas. The change of the grating's direction can provide an image with fluid and dynamic effects. The change of the grating's density can provide an image with colorful effect. The above two factors determine the effect of a hologram. However, the quality of image is determined by the resolution of a hologram. The lower the resolution of a hologram, the lower the density of pixels constituting an image. That is, the particles constituting an image are large, and thus the image is coarse.
A known method of fabricating dot matrix color hologram is disclosed by Taiwanese Patent No. 263565, which is a system for producing a digital color hologram. Referring to FIG. 3, in such a system, a laser beam 32a is generated by a laser light source 32. The laser beam 32a passes through a shutter controlled by a computer 31, then enters an interference system 35 including a beam splitter 38, and is split into two beams. The beam that maintains the original direction projects onto the recording substrate 39 after passing through the reflectors 38b, 40b and the triangular reflector 40c. The other beam 35a projects onto the recording substrate 39 after passing through the beam splitter 38a, the reflectors 38c, 38d, 40a and the triangular reflector 40c. The two beams having an identical optical path will interfere with each other. The interference fringes are then recorded on the recording substrate 39. The exposure time can be controlled by the shutter 33. The computer 31 can control the driver 50 to drive the revolving spindle 51 so as to rotate the interference system 40. The recording substrate 39 is placed on a turntable 36 and a 2-dimensionally-movable platform 37, Is so it can rotate and shift in a plane. In a system described above, a dot matrix color hologram can be obtained by forming gratings of different fringe densities. However, since the diameter of the spot of a laser beam is normally larger than 1 mm, i.e. 25.4 dpi, after being emitted from a laser cavity, and the conventional system can not reduce the diameter of the spot, therefore the particles of each pixel on the hologram look very clear. In other words, even though the image quality of the hologram is coarse, an excellent image is provided.